Method and device for temperature control in a combustion plant

ABSTRACT

A method for cooling and eliminating temperature variations in a power plant for combustion of a fuel in a pressurized fluidized bed includes pressuring air in a compressor and supplying the pressurized air to the pressurized fluidized bed through air paths after the pressurized air is cooled and temperature variations in the pressurized air are substantially eliminated prior to supplying the pressurized air into the pressurized fluidized bed in at least one transfer surface provided in the air paths. The at least one heat transfer surface is connected with a high temperature section of a feedwater/steam section for utilizing energy extracted in the heat transfer surface and the temperature variations of the pressurized air supplied to the pressurized fluidized bed are eliminated by controlling the feedwater/steam flow through the heat transfer surface based on the deviations between a desired temperature of the air to be delivered to the fluidized bed and a measured temperature of the pressurized air from the compressor.

TECHNICAL FIELD

The present invention relates to limitation of temperature variations inflowing gases in a combustion plant in which heat transfer surfaces arearranged in the gas paths to limit the temperature of the gas which issupplied to a combustor located in the plant and of the flue gasesemitted from the plant. The invention is especially valuable in a powerplant with combustion in a pressurized fluidized bed, a PressurizedFluidized Bed Combustion (PFBC)--plant, in which it permits limitationof temperature variations in pressurized air supplied to the combustorand flue gases emitted from the plant. This means that power output orefficiency remains essentially unaffected by variations in ambienttemperature and compression ratios.

BACKGROUND OF THE INVENTION

During combustion in a fluidized bed, the fluidized bed is supplied withair for fluidization of the bed material and for combustion of fuelsupplied to the fluidized bed. If the fluidized bed is part of a plantfor combustion in a pressurized fluidized bed, a PFBC plant, thefluidized bed contained within a bed vessel is enclosed in a pressurevessel and the air supplied to the fluidized bed is pressurized, forexample in a compressor driven by a gas turbine.

The mass flow of pressurized air supplied to a PFBC plant is controlledwithin an interval of 40-105% of nominal flow. The pressurization isnormally carried out in a gas turbine-driven compressor. From the pointof view of capital cost, high compression ratios are desirable A gasturbine-driven compressor provides different possibilities ofcontrolling the mass flow, depending on the type of gas turbine. Asingle-shaft unit may control the mass flow by varying the adjustment ofcompressor guide vanes and inlet valves, and, in addition, compressedair may be recirculated through the compressor. Moreover, in amulti-shaft unit, adjustable turbine guide vanes and nozzles as well asvariable rotor speed are utilized.

The temperature of the air supplied from the compressor via the pressurevessel to the fluidized bed must be limited, both when the air is usedfor cooling of pressure vessel, bed vessel, cyclones and othersupporting components arranged in the pressure vessel, and whentemperature variations, caused by compression ratios and ambienttemperature, in air supplied to the fluidized bed affect the outputpower from the plant and the efficiency of the plant.

The temperature of air supplied to the pressure vessel is not limited innormal PFBC plants, and thus there is no equalization of the temperaturevariations which occur in the pressurized air. Temperature variationsoccur as a consequence of variations in the ambient temperature andvarying compression ratios and are compensated for in a normal PFBCplant by a change in the output power from the plant and in theefficiency of the plant.

The residual heat in flue gases emitted from a combustion plant isdelivered to flue gas economizers, which are arranged in the flue gaspaths.

SUMMARY OF THE INVENTION

The influence of variations in the ambient temperature, compressionratios in air pressurized in the compressor, and the like, which in aplant for combustion in a pressurized fluidized bed, a PFBC--plant, isreflected in the output power from the plant and in the efficiency ofthe plant, is essentially eliminated when temperature variations inincoming Combustion air are limited according to the present invention.

The plant comprises a combustor in the form of a pressurized fluidizedbed, air paths in which air supplied to the fluidized bed ispressurized, flue gas paths in which energy contained in flue gasesemitted from the plant is partially extracted with a gas turbinearranged in the flue gas paths, and a feedwater/steam system comprisingheat transfer surfaces arranged in the air and flue gas paths.

According to the present invention, the temperature variations ofpressurized air supplied to the fluidized bed are limited by means ofheat transfer surfaces, preferably in the form of a heat exchanger,arranged in the air paths.

According to a preferred embodiment of the present invention, thetemperature of flue gases discharged from the plant is simultaneouslylimited with heat transfer surfaces, arranged in the flue gas paths, inthe form of cold and hot flue gas economizers. In addition, heattransfer surfaces arranged in the hot and cold sections of the flue gaspaths and in the air paths are interconnted in the high temperaturesection of the feedwater/steam system of the combustion plant. By thisinterconnection and by the arrangement of control valves adjacent to theheat transfer surfaces, for control and distribution of the heat work inand between the heat transfer surfaces, the temperature of air suppliedto the pressure vessel may be limited and maintained independent oftemperature variations of air pressurized in the compressor while at thesame time the flue gas temperature is limited.

The heat work in the heat transfer surfaces may be controlled fromoutside with temperature sensors, for example thermocouples, measuringtemperatures of air and flue gas, respectively. Measured temperaturesare compared, in conventional temperature regulators, with a desiredvalue and the deviation gives a control signal out from the temperatureregulator to the control valves arranged adjacent to the heat transfersurfaces. Based on the received control signal, the heat work in theheat-transfer surfaces is controlled.

Thus, according to the present invention, the necessary limitation ofthe temperature variations of air supplied to the fluidized bed isobtained, so that the output power from the combustion plant or theefficiency of the plant remains unaffected by ambient temperature andcompression ratios while at the same time heat absorbed in the heattransfer surfaces is utilized in the feedwater/steam system of the plant

In addition, during start-up and shutdown of a PFBC plant with controlof air and flue gas temperatures according to the present invention,possibilities are provided of reducing the heating and cooling times.

The heating time during start-up can be reduced and hence the corrosion,caused by flue gas condensate in the gas paths, be reduced by the heattransfer surfaces upon start-up being traversed by steam from anexternal source, for example from an existing auxiliary boiler intendedto supply the plant with de-aired water.

The cooling times can be reduced by the heat transfer surfaces, uponshutdown, being traversed by water, for example by being connected to acondenser circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will be explainedin greater detail with reference to functional and schematic flowdiagrams, wherein:

FIG. 1 illustrates a system for the limitation, in a combustion plantwith gas turbine-driven pressurization of air supplied to the combustor,of temperature variations of pressurized air supplied to the fluidizedbed in accordance with the present invention;

FIG. 2 shows schematically the parts of the air and flue gas paths, thefeedwater/steam system and other components of the plant, which arenecessary for the present invention;

FIG. 3 illustrates alternative solutions to the supply of thepressurized air to the pressure vessel;

FIGS. 4 and 5 show respectively, design and connection of thefeedwater/steam system to an auxiliary boiler during start-up and to acondenser circuit during cooling; and

FIG. 6 shows an alternative connection which under specialcircumstances, especially when only part of the pressurized air passesthe heat transfer surfaces in the air paths, provides increasedefficiency.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A system for limitation of temperature variations of pressurized airsupplied to the fluidized bed according to the present invention isschematically illustrated in FIG. 1. The air is supplied to a combustor10, in the form of a fluidized bed, through air paths 1, flue gasesformed during the combustion are discharged through flue gas paths 2 andheat is extracted from the plant and utilized through a feedwater/steamsystem 3.

In a power plant with combustion in a pressurized fluidized bed, aPressurized Fluidized Bed Combustion PFBC plant, the combustion takesplace in a fluidized bed 10 contained within a bed vessel 12 enclosed ina pressure vessel 11. Air is introduced into the plant at A, ispressurized in a compressor 13, the temperature being raised to atemperature which depends on the prevailing compression ratio and theambient temperature. The pressurized air is used for fluidization of thefluidized bed 10 and for combustion of fuel supplied to the fluidizedbed 10.

The flue gases formed during the combustion pass through a gas turbine14 arranged in the flue gas paths 2 of the plant, in which at least partof the energy contained in the flue gases is extracted. The compessor 13is suitably driven by the gas turbine 14. In addition, to increase theefficiency of the plant, the residual heat is extracted from the fluegases in heat transfer surfaces 15, 16, arranged in both the hot andcold sections of the flue gas paths 2, for example flue gas economizers,designated as the hot flue gas economizer 15 and the cold flue gaseconomizer 16, respectively, before the flue gases are discharged fromthe plant at B.

In order not to subject the pressure vessel 11 or other components,arranged in the pressure vessel 11 or the bed vessel 12, to hightemperatures, these are cooled by supplied pressurized air. To limit thetemperature of the supplied pressurized air and to correct fortemperature variations, caused by ambient temperature and compressionratios, the pressurized air passes through heat transfer surfaces 17,for example a heat exchanger, arranged in the air paths 1 beween thecompressor 13 and the pressure vessel 11. The temperature variations,which are caused by fluctuating ambient temperature or compressionratios, are corrected according to the present invention in the heatexchanger 17, which means that the efficiency of the plant is notaffected by these temperature fluctuations while at the same time energyextracted in the heat exchanger 17 is utilized in the feedwater/steamsystem 3 of the plant.

The temperature of the pressurized air is measured in conventionalmanner, for example by thermocouples 50, in the air paths downstream ofthe compressor 13. The measured temperature is compared with the desiredtemperature in a conventional temperature regulator 51. The deviationgives rise to an output signal, control signal supplied, to a controlvalve 18 by signal lines 52. The control valve 18 controls the heat workin the heat exchanger 17 by varying the flow of feedwater/steam throughthe heat exchanger 17, for example via the by-pass duct 19.

Variations in the feedwater/steam temperature arising downstream of theheat exchanger 17 are measured in conventional manner for example, bythermocouples 53 and corrected when the hot flue gases, in the hot fluegas economizer 15, pass through the feedwater/steam system 3 resultingin the flue gas temperature downstream of the hot flue gas economizer 15being influenced.

The influence on the flue gas temperature downstream of the hot flue gaseconomizer 15 is measured in conventional manner for example, by thethermocouples 54 and, after treatment in a conventional temperatureregulator 55, supplies a control signal to a control valve 20 by signalline 56. The control valve 20 then controls the heat work in the coldflue gas economizer 16, for example by distributing the feedwater/steamflow between the two branches 21 of the feedwater/steam circuit 3,comprising the cold flue gas economizer 16, and 22, comprising heattransfer surfaces 23 for heating another medium, for example highpressure feedwater. Where necessary or if the branch 22 is missing,feedwater/steam is conducted, at least partially, past the cold flue gaseconomizer 16, preferably via a by-pass duct 24.

By integration of the heat transfer surfaces 15, 16, 17, arranged in theair paths 1 and the flue gas paths 2, into the feedwater/steam system 3of the power plant, the invention provides a limitation of thetemperature of compressed air supplied to the pressure vessel and thebed vessel while at the same time temperature variations in this air areessentially eliminated. This means that the efficiency and power outputof the plant remain essentially unaffected by variations in ambienttemperature and compression ratios.

Energy extracted from air and flue gases is transferred to thefeedwater/steam system 3 of the power plant. The heat transfer surfaces15, 16, 17, which are necessary according to the present invention, areconnected at the point C, for example to a feedwater tank, and at thepoint D, for example to a boiler arranged in the fluidized bed 10, tothe high temperature section of the feedwater/steam system 3. In certainsituations, for example during start-up and shutdown of the power plant,the heat transfer surfaces may be connected to a circuit by beinginterconnected at C and D. If the circuit is then provided with steam orcold water, heating and cooling, respectively, of air paths 1 and fluegas paths 2 may be obtained.

FIG. 2 schematically shows how the heat transfer surfaces, which arenecessary for the present invention, are arranged in the air paths 1,flue gas paths 2 and feedwater/steam system 3 of the power plant.

In a PFBC plant pressurized air is supplied to a fluidized bed 10enclosed in a pressure vessel 11. The air is supplied to the fluidizedbed 10 for fluidization of the bed material and for combustion of fuelsupplied to the fluidized bed 10. The air, which is admitted from theenvironment via at least one controllable throttle valve 25, ispressurized in a compressor 13, suitably driven by a gas turbine 14arranged in the flue gas paths. The gas turbine 14 also drives agenerator 26. The gas turbine 14 and the compressor 13 are oftenintegrated into one unit and may be of an arbitrary type with a variablenumber of shafts. The figures show no intermediate cooling of thepressurized air, which occurs in multi-shaft units.

The mass flow of pressurized air to the pressure vessel 11 in a PFBCplant is controlled within an interval of 40-105% of nominal flow. Themass flow from the compressor 13 may, depending on the type of gasturbine/compressor unit 14/13, be controlled in different ways Asingle-shaft gas turbine/compressor unit 14/13, as indicated in FIG. 2,may be controlled by adjusting the throttle valve 25, the compressorguide vanes 27 and via a recirculation circuit 28 for pressurized air.For a multi-shaft gas turbine/compressor unit, the possibilities ofvarying turbine guide vanes, turbine nozzles and rotor speed are added.

The temperature of the pressurized air usually amounts to 350-450° C.,depending on compression ratio and ambient temperature. Before thepressurized air is supplied to the pressure vessel 11, it is cooled to atemperature suitable for the pressure vessel 11 and the parts enclosedin the pressure vessel 11, usually 200-300° C., in at least one heatexchanger 17 arranged in the air paths. According to the invention, theheat exchanger 17 is arranged in the high temperature section of thefeedwater/steam system 3, up-stream of a flue gas economizer 15 arrangedin the hot part of the flue gas paths 2.

To maintain the temperature of pressurized air supplied to the pressurevessel 11 essentially independent of compression ratio and ambienttemperature, the feedwater/steam flow through the heat exchanger 17 iscontrolled in a control valve 18. The control valve 18 distributes thefeedwater/steam flow, between the heat exchanger 17 and a by-pass duct19, based on the deviation between desired and measured temperature ofthe pressurized air. With the by-pass duct 19, the feedwater/steam flowis adapted to the measured temperature of the pressurized air. Withoutthe by-pass duct 19, there would be a risk of the feedwater temperatureand hence the temperature of air supplied to the pressure vessel 11dropping towards the ambient temperature.

The control in the heat exchanger 17 gives rise to variations of thefeedwater/steam temperature downstream of the heat exchanger 17, whichare essentially eliminated in at least one flue gas economizer 15arranged in the hot section of the flue gas paths 2, resulting in theflue gas temperature downstream of the hot flue gas economizer 15 beingaffected. The influence on the flue gas temperature is essentiallyeliminated in at least one flue gas economizer 16 arranged in the coldsection of the flue gas paths 2 by adapting the feedwater/steam flowtherethrough to correct, in conventional manner, any deviation, measuredin the flue gas paths 2 downstream of the hot flue gas economizer 15, ofthe flue gas temperature relative to the desired flue gas temperature.

The control of the feedwater/steam flow through the cold flue gaseconomizer is performed with the control valve 20 which controls thedistribution between the two parallel branches 21 and 22 in thefeedwater/steam system 3, including the cold flue gas economizer 16 andthe heat exchanger 23, respectively, connected for heating of anothermedium, for example high-pressure feedwater.

With heat transfer surfaces comprising at least one heat exchanger 17arranged in the air paths, in which the temperature of air supplied tothe pressure vessel 11 and the fluidized bed 10 is limited andtemperature variations in the air are essentially eliminated, at leastone flue gas economizer 15 arranged in the hot section of the flue gaspaths, in which simultaneously with the flue gas temperature beingreduced temperature variations of the feedwater/steam are essentiallyeliminated by allowing the flue gas temperature downstream of the hotflue gas economizer 15 to vary, at least one flue gas economizer 16arranged in the cold section of the flue gas paths, in which variationsof the flue gas temperature are essentially eliminated, and the by-passducts 19 and 24 for control of the heat work in the heat exchanger 17and the cold flue gas economizer 16, respectively, according to thepresent invention a limitation of the temperature of air supplied to thepressure vessel 11 and of flue gases emitted from the PFBC plant isobtained while at the same time the influence from ambient temperatureand compression ratios on the efficiency or the power output of theplant is essentially eliminated.

The heat exchanger 17 can be dimensioned for two cases:

I. Maximum heat work for the operation at the maximum air temperatureand full air flow;

II. Only part of the heat work of the operation, which means that partof the pressurized air is conducted past the heat exchanger 17 in a pipe29 direct to the air inlet to the fluidized bed 10.

The two cases are illustrated in FIG. 3.

Case I corresponds well with the previous description whereas in case IIonly part of the air quantity from the compressor 13 passes through theheat exchanger 17. The remaining air quantity is supplied, via a pipe29, to the cooled air flow near the air inlet to the fluidized bed 10.The distribution of air is controlled such that the heat work in theheat exchanger 17 is maintained constant, that is, an increased ambienttemperature entails an increased flow via the pipe 29. Case II meansthat the temperature of vital components such as pressure vessel 11, bedvessel 12 and cyclones 30 may be limited with a heat exchanger 17 oflimited power.

During start-up of a PFBC plant, air paths 1 and flue gas paths 2 arepreheated according to FIG. 4. Preheating is usually performed byburning fossil fuels in the air paths 1 upstream of the fluidized bed10. To avoid corrosion connected with flue gas condensate, componentsincluded in the air paths 1 and the flue gas paths 2 must be preheated,for example with dry hot air, to a temperature exceeding the dew pointof the flue gases which occur during the preheating. This first phase ofthe preheating is achieved in a favorable way by connecting the heattransfer surfaces,--the heat exchanger 17, the hot flue gas economizer15 and the cold flue gas economizer 16,--, which according to theinvention are interconnected and arranged in the air paths 1 and theflue gas paths 2, to an external source (not shown) with hot medium, forexample a boiler present in the plant and intended to supply the plantwith de-aired water during the start-up stage.

During the starting period the gas turbine 14 is driven by a startingdevice 31, which may consist of a frequency convertor which permits thegas turbine 14 to be run as a synchronous motor, but may also consist ofa motor connected to any of the shafts of the gas turbine 14, or otherstarting equipment for gas turbines The air is heated in the heatexchanger 17, the hot flue gas economizer 15 and the cold flue gaseconomizer 16 and transfers the heat to walls and other components inthe air paths 1 and the flue gas paths 2. If the bed vessel 12 is emptyand the valve 32 shown in FIGS. 2 and 3 is open, the air will flowthrough the pressure vessel 11 and the bed vessel 12 thus heating these.

The heat exchanger 17, the hot flue gas economizer 15 and the cold fluegas economizer 16 are connected in a starting circuit, which isillustrated in FIG. 4. As before, the heat transfer surfaces 15, 16, 17are connected to the high temperature section of the feedwater/steamsystem 3 of the plant, for example at an existing feedwater tank 33. Thefeedwater tank 33 is provided with steam, for example from an auxiliaryboiler (not shown) present in the plant. The feedwater/steam circulatesduring the starting stage from the feedwater tank 33 through the twoflue gas economizers 15 and 16 and the heat exchanger 17 and back to thefeed-water tank 33 via the open return pipe 34.

During shutdown of the plant, the cooling period can be shortened byutilizing the heat transfer surfaces 15, 16 and 17 arranged in the airpaths 1 and the flue gas paths 2 according to the invention. This makesthe plant more rapidly available for, for example, maintenance work. Theheat transfer surfaces 15, 16 and 17 are connected (see FIG. 5) to anexternal source with a coolant, for example a condenser circuit locatedin the plant for hot water production, via a valve 35. This causes theheat transfer surfaces 15, 16 and 17 arranged in the air paths 1 and theflue gas paths 2 to be traversed by a cold medium and the temperature inair and flue gas paths to be rapidly reduced.

An alternative solution of the arrangement of the heat exchanger 17 inthe system, in relation to the hot flue gas economizer 15, is shown inFIG. 6. The heat exchanger 17 is connected in parallel with the hot fluegas economizer 15, which reduces the temperature difference between airand feedwater/steam in the heat exchanger 17. Especially whendimensioning the heat exchanger 17 in accordance with the above case II,this solution may further increase the efficiency of the plant.

We claim:
 1. A method for cooling and eliminating temperature variationsin a power plant for combustion of a fuel in a pressurized fluidizedbed, said method comprising the steps of:pressuring air in a compressor;supplying the pressurized air to the pressurized fluidized bed throughair paths; cooling the pressurized air and substantially eliminatingtemperature variations in the pressurized air prior to supplying thepressurized air into the pressurized fluidized bed in at least one heattransfer surface provided in the air paths; connecting said at least oneheat transfer surface with a high temperature section of afeedwater/steam section for utilizing energy extracted in said heattransfer surface; eliminating the temperature variations of thepressurized air supplied to the pressurized fluidized bed by controllingthe feedwater/steam flow through said heat transfer surface based on thedeviations between a desired temperature of the air to be delivered tothe fluidized bed and a measured temperature of the pressurized air fromthe compressor.
 2. A method of cooling and eliminating temperatureaccording to claim 1, wherein said at least one heat transfer surface isa heat exchanger.
 3. A method of cooling and eliminating temperaturevariations according to claim 2, wherein said cooling and eliminatingtemperature variations includes measuring the temperature of thepressurized air from said compressor with temperature sensing means,comparing said measured temperature with said desired temperature intemperature regulating means, and based on a resulting temperaturedeviation supplying a control signal to a control valve means forcontrolling said flow of the feedwater/steam through said heatexchanger.
 4. A method of cooling and eliminating temperature variationsaccording to claim 2 further including the steps of:connecting at leastone heat transfer surface provided in flue gas paths to the hightemperature section of the feedwater/steam system; cooling andeliminating temperature variations in flue gases by said heat transfersurfaces in said flue gas paths by controlling and distributing the heatwork between said heat transfer surfaces arranged in said air and fluegas paths.
 5. A method of cooling and eliminating temperature variationsaccording to claim 4, wherein said controlling and distributing is basedon temperature deviations measured in said flue gas paths.
 6. A methodof cooling and eliminating temperature variations according to claim 4,wherein said heat transfer surfaces include at least one hot flue gaseconomizer and at least one cold flue gas economizer and whereinvariations in the feedwater/steam temperature downstream of said heatexchanger are substantially eliminated in said hot flue gas economizerconnected to the hot section of the flue gas paths, and whereinvariations in flue gas temperature are substantially eliminated in saidcold flue gas economizer arranged in the cold section of the flue gaspaths by controlling the feedwater/steam flow through the cold flue gaseconomizer based on temperature deviations measured in said flue gaspath downstream of said hot flue gas economizer.
 7. A method of coolingand eliminating temperature variations according to claim 2, wherein atleast part of the pressurized air supplied to the pressurized fluidizedbed by-passes said heat exchanger.
 8. A method of cooling andeliminating temperature variations according to claim 6, wherein atleast part of the pressurized air supplied to the pressurized fluidizedbed by-passes said heat exchanger.
 9. In a power plant for combustion ofa fuel in a pressurized fluidized bed, enclosed in a pressure vessel,apparatus for controlling temperature within said bed comprising:airpaths and a compressor arranged in the air for pressurizing air suppliedto the fluidized bed; flue gas paths and a gas turbine arranged in theflue gas paths for partially extracting energy contained in the fluegases; a feedwater/steam system interconnecting between said air andflue gas paths; means for limiting temperature of the compressed air toa desired temperature and for substantially eliminating variations inthe compressed air temperature said means including:a) at least one heattransfer surface arranged in said air paths, between said compressor andsaid pressure vessel, said heat transfer surface also being connected toa high temperature section of the feedwater/steam system; b) means forcontrolling a flow of feedwater/steam through said heat transfer surfacebased on temperature deviations between said desired temperature and ameasured temperature of the pressurized air from the compressor; c)means for measuring the temperature of the pressurized air from thecompressor; and d) means for comparing said measured temperature withsaid desired temperature and based on the resulting deviations supplyinga control signal to said controlling means.
 10. An apparatus accordingto claim 9, wherein said at least one heat transfer surface is a heatexchanger.
 11. An apparatus according to claim 10, furthercomprising:means for cooling and substantially eliminating temperaturevariations in said flue gas paths including at least one heat transfersurface arranged in said flue gas paths and connected to said hightemperature section of the feedwater/steam system.
 12. An apparatusaccording to claim 10, wherein heat transfer surfaces are arranged insaid flue gas paths to include at least one hot flue gas economizer andat least one cold flue gas economizer and wherein variations in thefeedwater/steam temperature downstream of said heat exchanger aresubstantially eliminated in said hot flue gas economizer connected inthe hot section of the flue gas paths.
 13. An apparatus according toclaim 12 further comprising means for controlling the feedwater/steamflow through the cold flue gas economizer based on temperaturedeviations measured in said flue gas path downstream of said hot fluegas economizer to substantially eliminate variations in flue gastemperature.
 14. An apparatus according to claim 10, wherein at leastpart of the pressurized air supplied to the pressurized fluidized bedby-passes said heat exchanger.